1. Field of the Invention
The present invention relates to processes for the electro-plating of a printed circuit board and, more particularly, to processes for facilitating the electro-gold-plating of a printed circuit board which has necessary circuit elements and circuit patterns provided by etching a surface of a copper clad laminate substrate. Some of the circuit patterns conduct signals only between the circuit elements on the same circuit board, but do not have electrical connection with the exterior of the circuit board.
2. Description of Prior Art
A circuit pattern on a printed circuit board is generally composed of a number of electrically independent conductive segments. In a conventional process for electro-plating predetermined portions of the independent conductive segments, some peripheral conductive patterns are provided from the conductive segments to the periphery of the circuit board so as to surround the periphery. One of plating electrodes and the peripheral conductive patterns are connected to supply plating electric current, with the result that the predetermined portions of the conductive segments are plated with conductive material.
There have been known some methods to deposit desired conductive material on selected areas of the circuit board and to prevent the conductive material from piling on undesired areas, for example, printing a resist layer, pressing a mask down on the circuit board with using some jig, or providing some contrivance for applying plating solution only to the certain areas. Anyway, the circuit board is soaked in an appropriate electrolyte bath. The plating solution bath and a certain point on the peripheral conductive patterns functions as a pair of electrodes. An electric current flows from an outside power source through therebetween to deposit desired conductive material on the selected areas of the circuit board.
However, there have been a problem. That is, the whole circuit board is dipped into the electrolyte solution in the conventional plating step, therefore a large quantity of electrolyte solution is necessary. Further, there is high possibility that some undesired portions of the circuit board and the jig are unnecessary plated, or that a little electrolyte solution is taken away in accordance with pulling out the circuit board from the electrolyte solution after plating step, resulting a waste of expensive plating material such as gold.
In addition, the electric current from the outside power source is supplied by only one pair of electrodes and flows thorough all independent conductive segments. The total length of the circuit is quite long and the circuit configuration is complicated, therefore electric resistance may be uneven at each conductive segments. As a result, the thickness of the deposited conductive material becomes inappropriately irregular.
By the way, there is a case where any peripheral conductive pattern can not be extended from a desired independent conductive segment to the periphery of the circuit board because of the desired circuit configuration. In this case, the desired independent conductive segment is temporally connected to another close conductive segment by an additional conductive pattern, so that an electric current flows from a peripheral conductive pattern through the desired conductive segment. Such an additional conductive pattern, practically, which is unnecessary in the final printing circuit configuration, is called as a "plating lead pattern". The plating lead patterns are only necessary as an intermediate step in the plating step of the printed circuit board. The desired primary conductive segments must again be electrically isolated. Thus, the electrical interconnection between the independent conductive segments must be removed before the printed circuit board is finished.
A prior art method of isolating the independent conductive segments from each other is disclosed in FIG. 6, wherein the plating lead patterns are physically cut off by drilling holes in the printed circuit board with using a cutter or a drill. Thus, in FIG. 6, a numeral 20 indicates a printed circuit board, a numeral 21 indicates a conductive pattern, a shaded portion 22 indicates an area to be plated and a numeral 24 indicates a plating lead pattern. A circled portion B indicates a drilled hole where the electrical connection between the independent conductive segments has been removed. As can be readily appreciated, by simply cutting off the plating lead patterns with a hole drilling technique, there still remain cut off sections of the plating lead patterns which are exposed and remain in contact with the desired independent conductive segments of the printed circuit board. Thus, there remains a risk that these independent segments may be shorted out or connected each other if a cut off section unintentionally contacts with conductive material to bridge the drilled hole.
An additional problem occurs in any production process desiring to produce a printed circuit board of an extremely high density. In such a design, the space between the respective conductive lines requires a distinct limitation on the compacting of the circuit pattern. Utilizing a plating lead pattern to interconnect the independent conductive segments while permitting a drilling or cutting off of the lead line, which is part of the finishing step, clearly creates a significant design problem. The deposited primary conductive pattern must be designed so that any essential component of conductive material in the desired final pattern should not be endangered in being cut off through any drilling step. If the design suggests that the plating lead pattern must be removed in a space between desired conductive segments which is smaller than the diameter of a drill, for example, as shown by circle A in FIG. 6, an alternative partial step for removing the plating lead pattern, such as a grinding or an abrading step, would be necessary instead of the drilling or cutting off step.
In one conventional process for producing a printed circuit board, a plurality of independent conductive segments and a plurality of plating lead patterns, which interconnect the independent conductive segments, are created on an insulating substrate. This is accomplished, for example, by etching a surface of a copper-clad laminate substrate. Subsequently, predetermined portions of the conductive segments are plated electrically to deposit an additional conductive material. Finally, the plating lead patterns are partially removed by drilling or, if necessary, grinding to insulate the independent conductive segments each other, resulting the final configuration of the printed circuit board.